Electrical devices, such as power supplies, switching regulators, motor control-regulators, computer electronics, audio amplifiers, surge protectors, and resistance spot welders often require substantial bursts of energy in their operation. Capacitors are energy storage devices that are commonly used to supply these energy bursts by storing energy in a circuit and delivering the energy upon timed demand.
Typically, capacitors consist of two electrically conducting plates, referred to as the anode and the cathode, which are separated by a dielectric film. In order to produce capacitors with high energy storage, the dielectric films where the electrical charge is stored, have a high capacitance and voltage and a low leakage current. Al2O3, TiO2, TiCaO3, Ta2O5, and Nb2O5 are oxides with a high capacitance that have been commercialized for capacitors. In general, these oxides are utilized by sintering fine powders, which are in a crystalline phase.
Crystalline ceramic capacitors are limited by breakdown failure at small fractions of their ‘theoretical’ dielectric strength, and to a lesser degree, by low value of dielectric constant because crystalline ceramics have line- and boundary-defects that are not repairable. They can operate at electric fields that are at most 1% to 5% of ‘theoretical’.
Capacitors employing anode base metals, such as Al, Ti, Ta, and Nb, can be anodized in electrolyte and form an anodic oxide. The anodic oxides can form anodic dielectrics having structural homogeneity at the atomic level, as in a glass, and therefore can have low-leakage current. In oxidizing electrolyte, anode base metals have the ability to self-repair up to certain thicknesses. This can determine the upper voltage limitation (e.g., several hundred Volt) of an electrolytic capacitor. Up to such voltage effective self-repair is possible so that such capacitors can work reliably at high energy density.
It is desirable to improve a dielectric's operating field strength E in the energy density formula (½ ∈o ∈ Ē2), where ∈o is the permittivity of vacuum, ∈ is the dielectric constant and Ē is the electrical field strength.
High electric field strength-requires elimination of all E-reducing defects in a dielectric. This means removal of geometrical, structural and chemical defects throughout the entire dielectric. According to U.S. Pat. No. 5,211,832, and U.S. Pat. No. 6,755,959, a high operating field can be achieved only when there is effective self-repair of defects that may arise in the dielectric, otherwise leakage current can turn into a spark and a short-circuit path through the dielectric. To enable anodic self-repair, volume defects (pores, tensile-strained regions), area defects (grain or phase boundaries), line defects (dislocations) and point defects (impurity atoms, vacancies, interstitials) must be absent. Materials that come close to this requisite are oxide glasses, an example of which is anodic Ta2O5 dielectric. Even then, atomic defects cannot be eliminated entirely, but anodic repair can be effective in neutralizing defects as long as the defects are small and few.
In order to achieve homogenous glassy anodic film, the microstructure of the substrate to be anodized should also be homogenous. Best homogeneity is obtained in glass materials. The elimination of microscopic and macroscopic defects in the dielectric as well as in the substrate is important. Anodizing at DC voltages forms a uniform dielectric film on Al, Ti, Nb, and Ta metals. If the substrate material has defects, which are typically found in a common metal, the anodic film on those local defects could have inhomogeneities and/or discontinuities. Anodizing a substrate with few and small defects, for example a nanometer-grained nano-scale or amorphous phase, enables the formation of an oxide dielectric, which has a continuous and nearly perfect film over the whole surface.